Materials having fine-scale microstructures have long been recognized to exhibit technologically-attractive properties. Recently, there has been developed a new class of microstructure materials, known as “nanostructured materials”, which are characterized by ultra-fine grains or particles embedded in a matrix. In nanostructured materials, a high fraction of the material's atoms are located at the boundaries of the grains or particles in the matrix.
In the past, research involving nanostructured materials has focused on the synthesis and processing of nanostructured bulk materials. Currently, however, there is a growing interest in nanostructured coatings including thermal barrier coatings, hard coatings and superhard nanocomposite coatings. A nanocomposite coating is a coating having more than one solid phase, at least one of which is in the nanometer range. Recent attention has been directed to nanocomposites due to the unique properties exhibited by these materials.
Titanium nitride (TiN) has been used as a protective and decorative coating in a variety of industries due to its high hardness, enhanced wear- and corrosion-resistant capability and golden color. However, due in large part to their low oxidation resistance at high temperatures, TiN coatings have some limitations under high-speed and dry-machining conditions. Silicon has been added to TiN coatings to form a stable oxide, preventing the coatings from undergoing severe oxidation. Besides their improved thermal stability, Ti—Si—N nanocomposite coatings exhibit enhanced hardness (over 40 GPa) as compared to TiN coatings (about 20 GPa). Ti—Si—N nanocomposite coating films have been characterized by nanocrystalline titanium nitride grains or particles embedded in a silicon nitride matrix.
In the past, nanocomposite Ti—Si—N films have been successfully synthesized using chemical vapor deposition (CVD), plasma-induced CVD and physical vapor deposition (PVD) techniques, as well as other methods. Recently, attention has been drawn to the PVD method because its hazard-free deposition process is safer and more compatible with industry application as compared to other deposition techniques. RF- and DC-supplied reactive unbalanced magnetron PVD sputtering has reportedly been found an effective technique for the fabrication of Ti—Si—N coatings.
In recent years, research efforts have focused on the relationship between coating structure and deposition conditions as indicative of the enhanced physical characteristics of Ti—Si—N nanocomposite coatings. Physical, structural and mechanical properties of Ti—Si—N coatings were characterized and a new structural model was introduced to explain the hardness enhancement mechanism. However, tribological properties are as relevant to the performance of protective coatings as are physical, structural and mechanical properties.
The super-hardness of recently-developed nanocomposite materials is a combined result of the stabilities of their nanostructure and interface. Therefore, thermal stability of nanocomposites is crucial for the stability of their hardness. The hardness of nanocrystalline/amorphous materials depends on four factors: (i) microstructure grains (grain size and texture of nanocrystallites), (ii) the chemical composition of each phase, (iii) the residual stress of the coatings, and (iv) the substrate temperature during coating deposition.
Recent research has shown the feasibility of synthesizing two-phase nanocomposite films with microstructures including nanocrystalline grains in an amorphous matrix. For example, nanocomposite TiN and SiNx films with hardness of 30–40 GPa have been synthesized by plasma-enhanced chemical vapor deposition (PECVD) and reactive magnetron sputtering. However, it has been found that the addition of controlled quantities of Si into a growing TiN film creates a diffusive barrier which blocks dislocations and suppresses plastic deformation. The result is a Ti—Si—N nanocomposite film with improved mechanical properties.